We present an extensive and up-to-date catalog of Galactic Cephei stars. This catalog is intended to give a comprehensive overview of observational characteristics of all known Cephei stars, covering information until 2004 June. Ninety-three stars could be confirmed to be Cephei stars. We use data from more than 250 papers published over the last nearly 100 years, and we provide over 45 notes on individual stars. For some stars we reanalyzed published data or conducted our own analyses. Sixty-one stars were rejected from the final Cephei list, and 77 stars are suspected to be Cephei stars. A list of critically selected pulsation frequencies for confirmed Cephei stars is also presented. We analyze the Cephei stars as a group, such as the distributions of their spectral types, projected rotational velocities, radial velocities, pulsation periods, and Galactic coordinates. We confirm that the majority of the Cephei stars are multiperiodic pulsators. We show that, besides two exceptions, the Cephei stars with high pulsation amplitudes are slow rotators. Those higher amplitude stars have angular rotational velocities in the same range as the high-amplitude Scuti stars (P rot k 3 days).We construct a theoretical HR diagram that suggests that almost all 93 Cephei stars are main-sequence objects. We discuss the observational boundaries of Cephei pulsation and the physical parameters of the stars. We corroborate that the excited pulsation modes are near to the radial fundamental mode in frequency and we show that the mass distribution of the stars peaks at 12 M . We point out that the theoretical instability strip of the Cephei stars is filled neither at the cool nor at the hot end and attempt to explain this observation.

Abstract. We present a near-infrared spectroscopic survey of a large area centered on the Cygnus OB2 association aimed at constraining its massive star contents. Our goal is to establish a nearly complete list of O-type members of the association, both to examine recent claims based on starcounts that suggest a richer content than previously thought, and to provide a suitable database for further studies of the entire high-mass end of one of the richest associations of the Galaxy. The target selection is based on the JHK photometry published in the 2MASS all-sky survey. We identify 46 new early-type candidates, most of them expected to be O-type stars, plus 16 new stars with emission in Brγ and often in other lines as well, characteristic of evolved massive stars undergoing intense mass loss. We also present spectra of three luminous stars with CO overtone emission, one of them having also intense H2 emission and being associated with compact nebulosity. By considering our findings, those of other authors, and plausible completeness corrections, we estimate the number of O-type stars or stars having evolved from a O-type progenitor to be 90-100, slightly below, but compatible with, most recent starcounts estimates by Knödlseder (2000, A&A, 360, 539). These results support the notion that Cygnus OB2 may be considered as a young globular cluster. The lists of new members that we provide, in particular those with emission lines, should be a useful resource for future investigations of Cygnus OB2 itself, as well as of very massive stellar evolution by providing a nearby, abundant sample of stars sharing a common environment.

Abstract. BPM 37093 is the only hydrogen-atmosphere white dwarf currently known which has sufficient mass (∼1.1 M ) to theoretically crystallize while still inside the ZZ Ceti instability strip (T eff ∼ 12 000 K). As a consequence, this star represents our first opportunity to test crystallization theory directly. If the core is substantially crystallized, then the inner boundary for each pulsation mode will be located at the top of the solid core rather than at the center of the star, affecting mainly the average period spacing. This is distinct from the "mode trapping" caused by the stratified surface layers, which modifies the pulsation periods more selectively. In this paper we report on Whole Earth Telescope observations of BPM 37093 obtained in 1998 and 1999. Based on a simple analysis of the average period spacing we conclude that a large fraction of the total stellar mass is likely to be crystallized.

Context. The Crab pulsar emits across a large part of the electromagnetic spectrum. Determining the time delay between the emission at different wavelengths will allow to better constrain the site and mechanism of the emission. We have simultaneously observed the Crab Pulsar in the optical with S-Cam, an instrument based on Superconducting Tunneling Junctions (STJs) with μs time resolution and at 2 GHz using the Nançay radio telescope with an instrument doing coherent dedispersion and able to record giant pulses data. Aims. We have studied the delay between the radio and optical pulse using simultaneously obtained data therefore reducing possible uncertainties present in previous observations. Methods. We determined the arrival times of the (mean) optical and radio pulse and compared them using the tempo2 software package.Results. We present the most accurate value for the optical-radio lag of 255±21 μs and suggest the likelihood of a spectral dependence to the excess optical emission asociated with giant radio pulses.

We present the results of a photometric multi-colour, two-site observing campaign of β Cephei stars in the young open cluster NGC 4755. More than 260 h of time-series U, B, V filter data of the cluster were acquired on two continents during 1999 and 2000. We list the detected pulsation frequencies and discuss the results of their analysis. Mode identification of the detected modes is performed as well.In addition, we present the detection of four new variable stars in NGC 4755. We give frequency solutions as indicators of the time-scales and amplitudes of the pulsations. NGC 4755-116 is probably a B2 dwarf with a period of 4.2 d whose variability is caused by a spot or g-mode pulsation. NGC 4755-405 can be considered as a new β Cephei star with two pulsation frequencies. For NGC 4755-215 we found one frequency and for NGC 4755-316 three pulsation frequencies; we suggest that both are new slowly pulsating B stars of short period.