HS0507+0434: a double DA degenerate with a ZZCeti component

HS0507+0434 and HS2240+1234 are two new common proper motion pairs of DA white dwarfs, discovered by the Hamburg Quasar Survey. Our model atmosphere analysis shows that HS0507+0434A has an effective temperature of 20000K and is remarkably young (tau_cool < 1Gyr) compared to the average cooling time of all known wide double degenerates (~ 3Gyr). The cooler B component of HS0507+0434 is particularly interesting; the determination of the atmospheric parameters is complicated by the strong dependence of the solution on the details of the treatment of convection. Only those parameterizations of mixing length theory are consistent with all observations (especially the magnitude difference between the components), which lead to an intermediate efficiency of the convective flux. In the standard version of the mixing length theory this corresponds to a mixing length parameter of l/H_p = 1.75 - 2.0, where H_p is the pressure scale height. This result does not depend on the model atmosphere code and is in agreement with previous studies of convection in DA white dwarfs; there are, however, slightly different formulations of the MLT in use, which achieve the same efficiency at different values of l/H. These versions are discussed and compared in the paper. The result of our analysis (T_eff=11900K, log g=8) places the B component into the ZZCeti instability strip, where DA white dwarfs are pulsating non-radially. Photometric observations have now confirmed that HS0507+0434 is variable and identified 3 or 4 fundamental g-modes in the Fourier spectrum. Based on observations collected at the German-Spanish Astronomical Center (DSAZ) at Calar Alto, Spain, at the European Southern Observatory, La Silla, Chile, with the International Ultraviolet Explorer Satellite, and at the Observatoire de Haute Provence (CNRS), France.