The high-field magnetic white dwarf LP 790-29: not a fast rotator

Abstract

We have investigated the nature of the magnetic white dwarf LP 790-29 = LHS 2293 by polarimetric monitoring, searching for short-term variability. No periodicity was found and we can exclude rotation periods between 4 sec and 1.5 hour with a high confidence. Maximum amplitudes of sinusoidal variations are Delta R < 0.009 mag and Delta V_R < 0.7% for a mean value of the R-band circular polarization of V_R = +9.1+/-0.3%. Combined with earlier results by other authors, our observation suggests that LP 790-29 is, in fact, an extremely slowly rotating single white dwarf and not an unrecognized fast rotator and/or disguised cataclysmic variable.