Apsidal motion of the eccentric eclipsing binary DI Herculis: An apparent discrepancy with general relativity

In 1959, Rudkjobing called attention to the 8th magnitude, eccentric eclipsing binary DI Herculis as an important test case for studying relativistic apsidal motion, since the theoretical relativistic apsidal motion is greater than that expected from the classical effects (i.e., from the tidal and rotational deformation of the stellar components). Excellent determinations of the orbital and stellar parameters of the system have been made by Popper (1982) from the combined analysis of the system's radial-velocity data and UBV light curves of Martynov and Khaliullin (1980), which permit the theoretical relativistic and classical components of the apsidal motion to be determined with reasonable certainty: omega-dot/sup theor//sub GR/ = 2X34/100 yr and omega-dot/sup theor//sub CL/ = 1X93/100 yr. Least-squares solutions of the timings of primary and secondary minima, extending over an 84-yr interval, and including eclipse timings obtained as recently as 1984, yield a small advance of periastron omega-dot/sup obs/ = 0X65/100 yr +- 0X18/100 yr. The observed advance of the periastron is about one-seventh the theoretical value of omega-dot/sup theor//sub GR+CL/ = 4X27/100 yr that is expected from the combined relativistic and classical effects, and results in a discrepancy of -3X62/100 yr, a value which has a magnitude of approx.20 sigma. Classical mechanisms that can possibly explain this apparent discrepancy are discussed, along with the possibility that there may be problems with general relativity