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

We present the first UBV and uvby photometric observations for the short pe riod 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 b inary nature of HV UMa. In order to reveal the real physical status of this star, we took medium resolution (lambda/Delta lambda approximate to 11000) spectra in the red spectral region centered at 6600 Angstrom. Spectra obta ined around the assumed quadratures clearly showed the presence of the seco ndary component. An improved ephemeris calculated using our and Hipparcos epoch photometry i s Hel. JD(min) = 2451346.743 +/- 0.001, P = 0(d).7107523(3). A radial veloc ity curve was determined by modelling the cores of Ha profiles with two Gau ssian components. This approximative approach gave a spectroscopic mass rat io of q(sp)=0.19+/-0.03. A modified Lucy model containing a temperature exc ess of the secondary was fitted to the V light curve. The obtained set of p hysical parameters together with the parallax measurement indicate that thi s binary lies far from the galactic plane, and the primary component is an evolved object, probably a subgiant or giant star. The large temperature ex cess of the secondary may suggest a poor thermal contact between the compon ents due to a relatively recent formation of this contact system.