Few observational constraints exist for the tidal synchronization rate of late-type stars, despite its fundamental role in binary evolution. We visually inspected the light curves of 2278 eclipsing binaries (EBs) from the Kepler Eclipsing Binary Catalog to identify those with starspot modulations, as well as other types of out-of-eclipse variability. We report rotation periods for 816 EBs with starspot modulations, and find that 79% of EBs with orbital periods of less than 10 days are synchronized. However, a population of short-period EBs exists, with rotation periods typically 13% slower than synchronous, which we attribute to the differential rotation of highlatitude starspots. At 10 days, there is a transition from predominantly circular, synchronized EBs to predominantly eccentric, pseudosynchronized EBs. This transition period is in good agreement with the predicted and observed circularization period for Milky Way field binaries. At orbital periods greater than about 30 days, the amount of tidal synchronization decreases. We also report 12 previously unidentified candidate δ Scuti and γ Doradus pulsators, as well as a candidate RS CVn system with an evolved primary that exhibits starspot occultations. For short-period contact binaries, we observe a period-color relation and compare it to previous studies. As a whole, these results represent the largest homogeneous study of tidal synchronization of late-type stars.

LP 876-10 is a nearby active M4 dwarf in Aquarius at a distance of 7.6 pc.The star is a new addition to the 10-pc census, with a parallax measured via the Research Consortium on Nearby Stars (RECONS) astrometric survey on the Small & Moderate Aperture Research Telescope System's (SMARTS) 0.9-m telescope. We demonstrate that the astrometry, radial velocity, and photometric data for LP 876-10 are consistent with the star being a third, bound, stellar component to the Fomalhaut multiple system, despite the star lying nearly 6 • away from Fomalhaut A in the sky. The 3D separation of LP 876-10 from Fomalhaut is only 0.77 ± 0.01 pc, and 0.987 ± 0.006 pc from TW PsA (Fomalhaut B), well within the estimated tidal radius of the Fomalhaut system (1.9 pc). LP 876-10 shares the motion of Fomalhaut within ∼1 km/s, and we estimate an interloper probability of ∼10 −5 . Neither our echelle spectroscopy nor astrometry are able to confirm the close companion to LP 876-10 reported in the Washington Double Star Catalog (WSI 138). We argue that the Castor Moving Group to which the Fomalhaut system purportedly belongs, is likely to be a dynamical stream, and hence membership to the group does not provide useful age constraints for group members. LP 876-10 (Fomalhaut C) has now risen from obscurity to become a rare example of a field M dwarf with well-constrained age (440 ± 40 Myr) and metallicity. Besides harboring a debris disk system and candidate planet, Fomalhaut now has two of the widest known stellar companions. Subject headings: binaries: visual -Stars: activity -Stars: fundamental parameters -Stars: individual (LP 876-10, Fomalhaut, TW PsA) -Stars: rotation sured. θ is the projected angular separation from Fomalhaut, and PA is the position angle as measured north through east. this paper, using positions from van Leeuwen (2007) and Roeser et al. (2010). In the Table and throughout the paper, Galactic velocities U and positions X are defined towards the Galactic center, V and Y are in the direction of Galactic rotation, and W and Z is towards the North Galactic pole.

We present the flare occurrence rates and starspot evolution for GJ 1245 A and B, two active M5 stars, based on nine months of Kepler short cadence observations, and four years of nearly continuous long cadence observations. The A component is separated from the B component by 7 , and the stars are not resolved in the Kepler pipeline processing due to Kepler's large plate scale of 4 /pixel. Analyzing the target pixel data, we have generated separate light curves for components A and B using the PyKE pixel response function modeling procedures, and note the effects of CCD saturation and non-linear response to high energy flares. In our sample, GJ 1245A and B exhibit an average of 3.0 and 2.6 flares per day, respectively. We introduce a new metric, L f l /L Kp , to compare the flare rates between stars, and discuss this in the context of GJ 1245 A and B. Both stars exhibit starspot features that evolve on long time scales, with the slower rotating B component showing evidence of differential rotation. Intriguingly, the angular separation between the A and B component photocenters decreases during the four years of observations in a manner consistent with a shift in the position of the A photocenter due to the orbit of its unseen M8 companion (GJ 1245C), which is ∼94% less bright. Among the most detailed photometric studies of fully convective M dwarfs in a multiple system, these results provide an important constraint on stellar age-rotation-activity models.

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We present an analysis of long-term photometric variability for nearby red dwarf stars at optical wavelengths. The sample consists of 264 M dwarfs south of Dec. = +30 with V − K = 3.96-9.16 and M V ≈ 10-20, corresponding to spectral types M2V-M8V, most of which are within 25 pc. The stars have been observed in the V RI filters for ∼4-14 years at the CTIO/SMARTS 0.9m telescope. Of the 238 red dwarfs within 25 pc, we find that only ∼8% are photometrically variable by at least 20 mmag (∼2%) in the V RI bands. Only four stars have been found to vary by more than 50 mmag, including GJ 1207 at 8.6 pc that experienced a single extraordinary flare, and GJ 2006A, TWA 8A, and TWA 8B, which are all young stars beyond 25 pc linked to moving groups. We find that high variability at optical wavelengths over the long-term can in fact be used to identify young stars. Overall, however, the fluxes of most red dwarfs at optical wavelengths are steady to a few percent over the long term.The low overall rate of photometric variability for red dwarfs is consistent with results found in previous work on similar stars on shorter timescales, with the body of work indicating that most red dwarfs are only mildly variable. As expected, we find that the degree of photometric variability is greater in the V band than in the R or I bands, but we do not find any obvious trends in variability over the long term with red dwarf luminosity or temperature. We highlight 17 stars that show long-term changes in brightness, sometimes because of flaring activity or spots, and sometimes because of stellar cycles similar to our Sun's solar cycle. Remarkably, two targets show brightnesses that monotonically increase (G 169-029) or decrease (WT 460AB) by several percent over a decade. We also provide long-term variability measurements for seven M dwarfs within 25 pc that host exoplanets, none of which vary by more than 20 mmag. Both as a population, and for the specific red dwarfs with exoplanets observed here, photometric variability is therefore often not a concern for planetary environments, at least at the optical wavelengths where they emit much of their light. ). These results and ours will be discussed in more detail in §8 to provide a portrait of the dominant stellar component of our Galaxy, the red dwarfs. SampleFor this study, we focus on a sample of 264 red dwarfs south of Dec. = +30 observed at the CTIO/SMARTS 0.9m, some starting as long ago as 1999. The sample is listed in Table 1, with stars falling in the ranges of V − K = 3.96-9.16 and M V ≈ 10-20, corresponding to spectral types M2V-M8V. Known cool subdwarfs have been removed, as they have been previously discussed in detail in Jao et al. (2011). Multiple systems with separations in the range 1-3 ′′ have been omitted because even if two sources can be seen in some images, accurate photometry for each source cannot always be determined, given variable seeing. Known multiples with separations less than 1 ′′ are treated as single sources and noted with component letters in Table 1, e.g., AB, o...