HV Ursae Majoris, a new contact binary with early-type components

We present the first UBV and uvby photometric observations for the short period variable star HV Ursae Majoris classified as a field RRc variable. The observed differences between the consecutive minima and the lack of colour variations disagree with the RRc-classification and suggest the possible binary nature of HV UMa. In order to reveal the real physical status of this star, we took medium resolution spectra in the red spectral region centered at 6600 A. Spectra obtained around the assumed quadratures clearly showed the presence of the secondary component. An improved ephemeris calculated using our and Hipparcos epoch photometry is Hel. JD_min=2451346.743+/-0.001, P=0.7107523(3) d. A radial velocity curve was determined by modelling the cores of Halpha profiles with two Gaussian components. This approximative approach gave a spectroscopic mass ratio of q_sp=0.19+/-0.03. A modified Lucy model containing a temperature excess of the secondary was fitted to the V light curve. The obtained set of physical parameters together with the parallax measurement indicate that this binary lies far from the galactic plane, and the primary component is an evolved object, probably a subgiant or giant star. The large temperature excess of the secondary may suggest a poor thermal contact between the components due to a relatively recent formation of this contact system.