We report the first extensive set of optical photometric observations of the counterpart to SAX J1808.4-3658 (V4580 Sagittarius) in quiescence. The source was detected at V~21, 5 magnitudes fainter than at the peak of its 1998 outburst. However, a comparable ~6% semi-amplitude 2hr modulation of its flux is revealed. This has the same phasing and approximately sinusoidal modulation as seen during outburst and with photometric minimum when the pulsar is behind the companion. The lack of a double-humped morphology rules out an ellipsoidal origin, implying that the bulk of the optical flux does not arise from the companion. Moreover, applying crude modelling to the disc and X-ray irradiated face of the donor shows that the internal energy release of a remnant disc (with mass transfer driven by gravitational radiation) is sufficient to explain most of the optical emission, and with the modulation due to the varying contribution of the heated star's face. We note that this model is also consistent with the much lower X-ray to optical flux ratio in quiescence versus outburst, and with the phasing of the optical modulation.Comment: 7 pages, 5 figures, accepted version with very minor changes (after ref's report), to appear in MNRA

The dwarf nova GW Librae (GW Lib) is the first cataclysmic variable (CV) discovered to have a primary in a white dwarf instability strip, making it the first multimode, non‐radially pulsating star known to be accreting. The primaries of CVs, embedded in hot, bright accretion discs, are difficult to study directly. Applying the techniques of asteroseismology to GW Lib could therefore give us an unprecedented look at a white dwarf that has undergone ∼109 yr of accretion. However, an accreting white dwarf may have characteristics sufficiently different from those of single pulsating white dwarfs to render the standard models of white dwarf pulsations invalid for its study. This paper presents amplitude spectra of GW Lib from a series of observing campaigns conducted during 1997, 1998 and 2001. We find that the dominant pulsation modes cluster at periods near 650, 370 and 230 s, which also appear in linear combinations with each other. The pulsation spectrum of GW Lib is highly unstable on time‐scales of months, and exhibits clusters of signals very closely spaced in frequency, with separations on the order of 1 μHz.

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.

We have used the Advanced Camera for Surveys on board the Hubble Space Telescope to image the core of the globular cluster M 15 in the far-ultraviolet (FUV) waveband. Based on these observations, we identify the FUV counterpart of the recently discovered low-mass X-ray binary M 15 X-2. Our time-resolved FUV photometry shows a modulation with 0.062 ± 0.004 mag semi-amplitude and we clearly detect a period of 22.5806 ± 0.0002 min. We have carried out extensive Monte Carlo simulations which show that the signal is consistent with being coherent over the entire observational time range of more than 3000 cycles. This strongly suggests that it represents the orbital period of the binary system. M 15 X-2 is FUV bright (FUV ≃ 17 mag) and is characterized by an extremely blue spectral energy distribution (F λ ∝ λ −2.0 ). We also find evidence for an excess of flux between 1500Å and 1600Å and probably between 1600Å and 2000Å which might be due to C IV 1550Å and He II 1640Å emission lines. We also show that M15 X-2's X-ray luminosity can be powered by accretion at the rate expected for gravitational-wave-driven mass transfer at this binary period. The observed FUV emission appears to be dominated by an irradiated accretion disk around the neutron star primary, and the variability can be explained by -2irradiation of the low-mass white dwarf donor if the inclination of the system is ≈ 34 • . We conclude that all observational characteristics of M15 X-2 are consistent with it being an ultracompact X-ray binary, only the third confirmed such object in a globular cluster.