The long-period orbit of the dwarf nova V630 Cassiopeiae

Abstract

We present extensive spectroscopy and photometry of the dwarf nova V630 Cassiopeiae. A late-type (K4-5) absorption spectrum is easily detectable, from which we derive the orbital parameters. We find a spectroscopic period of P = 2.56387 ± 4 × 10-5 d and a semi-amplitude of K2 = 132.9 ± 4.0 km s-1. The resulting mass function, which is a firm lower limit on the mass of the white dwarf, is then f(M) = 0.624 ± 0.056M⊙. The secondary star is a 'stripped giant' and, using relations between the core mass and the luminosity and between the core mass and the radius, we derive a lower limit of M2 ≥ 0.165 M⊙ for the secondary star. The rotational velocity of the secondary star is not resolved in our spectra and we place a limit of Vrot sin i < 40 km s-1. The long-term light curve shows variations of up to 0.4 mag on short (1-5 d) time-scales, and variations of 0.2-0.4 mag on longer (3-9 months) time-scales. In spite of these variations, the ellipsoidal light curve of the secondary star is easily seen when the data are folded on the spectroscopic ephemeris. Ellipsoidal models fitted to the mean light curve give an inclination in the range 66°.96 ≤ i ≤ 78°.08 (90 per cent confidence). This inclination range, and the requirement that M2 ≥ 0.165 M⊙ and Vrot sin i < 40 km s-1, yields a white dwarf mass of M1 = 0.977+0.168-0.098 M⊙ and a secondary star mass of M2 = 0.172+0.029-0.012 M⊙ (90 per cent confidence limits). Our findings confirm the suggestion of Warner that V630 Cas is a rare example of a dwarf nova with a long orbital period.