Period Change, Spot Migration and Orbital Solution for the Eclipsing Binary AR Lac

Abstract

We present a new large set of photoelectric photometric measurements (1806 in V and 1367 in B passbands) of the eclipsing RS CVn-type system AR Lac covering the period 2000 June 28 to 2005 October 13. 13 new epochs of minima have been derived that allows us to update the complex O--C pattern of this binary, now monitored for more than a hundred years. The O--C pattern over the last few decades is found to be characterized by a linear decreasing trend (amounting to anticipation by one full orbital revolution every 125 years) with a superimposed sinusoidal modulation of a 40 yr period. An improved orbital solution is obtained by modeling our B and V observations. The distance implied by the refined orbital solution is 41.6±1.9 pc which is within the Hipparcos error bar and differs from it (42.0±1.0 pc) by less than 1%. A new longitude migration rate is derived for the two-spot model usually adopted in modeling AR Lac. The value fitting our 2000--2005 observations is a migration rate toward decreasing orbital phases amounting to 0.55 period yr^-1, significantly faster than the 0.4 period yr^-1 derived in literature for observations covering the years 1978--1981.