We report the discovery of 16 detached M dwarf eclipsing binaries with J < 16 mag and provide a detailed characterization of three of them, using high-precision infrared light curves from the WFCAM Transit Survey (WTS). Such systems provide the most accurate and modelindependent method for measuring the fundamental parameters of these poorly understood yet numerous stars, which currently lack sufficient observations to precisely calibrate stellar evolution models. We fully solve for the masses and radii of three of the systems, finding orbital periods in the range 1.5 < P < 4.9 d, with masses spanning 0.35-0.50 M and radii between 0.38 and 0.50 R , with uncertainties of ∼3.5-6.4 per cent in mass and ∼2.7-5.5 per cent in radius. Close companions in short-period binaries are expected to be tidally locked into fast rotational velocities, resulting in high levels of magnetic activity. This is predicted to inflate their radii by inhibiting convective flow and increasing starspot coverage. The radii of the WTS systems are inflated above model predictions by ∼3-12 per cent, in agreement with the observed trend, despite an expected lower systematic contribution from starspot signals at infrared wavelengths. We searched for correlation between the orbital period and radius inflation by combining our results with all existing M dwarf radius measurements of comparable precision, but we found no statistically significant evidence for a decrease in radius inflation for longer period, less active systems. Radius inflation continues to exists in non-synchronized systems, indicating that the problem remains even for very low activity M dwarfs. Resolving this issue is vital not only for understanding the most populous stars in the Universe, but also for characterizing their planetary companions, which hold the best prospects for finding Earth-like planets in the traditional habitable zone.

We report on the discovery of four ultra‐short‐period (P ≤ 0.18 d) eclipsing M‐dwarf binaries in the Wide‐Field Camera (WFCAM) Transit Survey. Their orbital periods are significantly shorter than that of any other known main‐sequence binary system, and are all significantly below the sharp period cut‐off at P ∼ 0.22  d as seen in binaries of earlier‐type stars. The shortest‐period binary consists of two M4‐type stars in a P = 0.112  d orbit. The binaries are discovered as part of an extensive search for short‐period eclipsing systems in over 260 000 stellar light curves, including over 10 000 M‐dwarfs down to J = 18 mag, yielding 25 binaries with P ≤ 0.23 d. In a popular paradigm, the evolution of short‐period binaries of cool main‐sequence stars is driven by the loss of angular momentum through magnetized winds. In this scheme, the observed P ∼ 0.22 d period cut‐off is explained as being due to time‐scales that are too long for lower‐mass binaries to decay into tighter orbits. Our discovery of low‐mass binaries with significantly shorter orbits implies that either these time‐scales have been overestimated for M‐dwarfs, e.g. due to a higher effective magnetic activity, or the mechanism for forming these tight M‐dwarf binaries is different from that of earlier‐type main‐sequence stars.

Aims. We report the discovery of five new dwarf novae that were spectroscopically identified in the Hamburg Quasar Survey (HQS), and discuss the properties of the sample of new dwarf novae from the HQS. Methods. Follow-up time-resolved spectroscopy and photometry have been obtained to characterise the new systems. Results. The orbital periods determined from analyses of the radial velocity variations and/or orbital photometric variability are P orb 105.1 min or P orb 109.9 min for HS 0417+7445, P orb = 114.3 ± 2.7 min for HS 1016+3412, P orb = 92.66 ± 0.17 min for HS 1340+1524, P orb = 272.317 ± 0.001 min for HS 1857+7127, and P orb = 258.02 ± 0.56 min for HS 2214+2845. HS 1857+7127 is found to be partially eclipsing. In HS 2214+2845 the secondary star of spectral type M3 ± 1 is clearly detected, and we estimate the distance to the system to be d = 390 ± 40 pc. We recorded one superoutburst of HS 0417+7445, identifying the system as a SU UMatype dwarf nova. HS 1016+3412 and HS 1340+1524 have rare outbursts, and their subtype is yet undetermined. HS 1857+7127 frequently varies in brightness and may be a Z Cam-type dwarf nova. HS 2214+2845 is a U Gem-type dwarf nova with a most likely cycle length of 71 d. Conclusions. To date, 14 new dwarf novae have been identified in the HQS. The ratio of short-period (<3 h) to long-period (>3 h) systems of this sample is 1.3, much smaller compared to the ratio of 2.7 found for all known dwarf novae. The HQS dwarf novae display typically infrequent or low-amplitude outburst activity, underlining the strength of spectroscopic selection in identifying new CVs independently of their variability. The spectroscopic properties of short-period CVs in the HQS, newly identified and previously known, suggest that most, or possibly all of them are still evolving towards the minimum period. Their total number agrees with the predictions of population models within an order of magnitude. However, the bulk of all CVs is predicted to have evolved past the minimum period, and those systems remain unidentified. This suggests that those post-bounce systems have markedly weaker Hβ emission lines compared to the average known short-period CVs, and undergo no or extremely rare outbursts.