The donor star of the long-period dwarf nova DX Andromedae

Spectroscopic observations with the INT show that DX And has (i) a binary period of 0.44167 d (10.6 h), (ii) a secondary radial velocity amplitude of 107 km s–1and (iii)a projected rotational velocity of 79 km s–1. Photometric observations with the JKT in the BRZ passbands reveal ellipsoidal variation of 0.08 mag in Z. Taking all the data together we find a mass ratio of 0.96, that the accretion disc contributes around 15 percent of the light in the green region of the spectrum, and that the secondary has a spectral type of K1 V and a radius at least 40 per cent larger than the corresponding main-sequence star. Because three methods are used to determine the spectral type and are found to agree, ours is a more secure determination than is typical for this class of object. It is argued that the companion star mass is less than 0.8 M⊙, a value that is consistent with the binary inclination estimated from the ellipsoidal variation. The distance to DX And is 630 pc. The quality of the secondary star's absorptionline spectrum is sufficient to show for the first time that it does not exactly match that of a normal main-sequence star: enhanced absorption is seen at some wavelengths. Stellar activity, evolutionary modification of the surface abundances and gravity darkening are possible causes of the effect.