Discovery of apsidal motion in α Coronae Borealis by means of ROSAT X-ray eclipse timing

Four ROSAT X-ray observations of the secondary optical minimum of the eclipsing binary system α CrB taken in 1992, 1993 and 1997 are presented. Because of the totality of the X-ray eclipse, the times of mid eclipse can be accurately determined from the ROSAT data. The period between secondary minima Ps is found to be significantly different from the optically well determined period between primary minima Pp, thus indicating apsidal motion. The observed value Ps − Pp = 4.8 ± 2.1 seconds is, first, shown to be almost exclusively due to the primary component, and second, consistent with our current knowledge of α CrB A. The relativistic contribution to the observed value is 0.95 seconds or 17 % of the total effect.