DI Hercules relates to relativity.

One of the dangers of writing down a universal theory of gravity, as Einstein did with his general relativity, is that it lies open for any obscure object in the universe to detract from it. Edward F. Guinan and Frank P. Maloney of Villanova (Pa.) University now present "an apparent discrepancy with general relativity" in the motion of the binary star system DI Herculis, an eighth-magnitude (very dim) object that was not discovered until 13 years after Einsterin published his theory of general relativity.

Orbital motions are the classic testing grounds of theories of gravity. Precession of the orbit of the planet MErcury determined the superiority of Einsterin's theory over Newton's. Precession is a motion of the orbit as a whole: Not only does Mercury go around the orbit; at the same time the orbit itself swings around like the hand of a clock. In planets the latter motion is called precession of perihelion; in stars it is either precession of periastron or precession of apsides.

DI Herculis is an eccentric (highly elliptical) eclipsing system. Its orbit lies at such an angle that, viewed from earth, it has deep and narrow eclipses as one of the stars passes in front of the other. It is also fast, completing an orbit every 10.55 days. These characteristics make it favorable for such a test.

To calculate what the precession should be, Guinan and Maloney calculated the amount predicted by classical (Newtonian) theory as 1.93[deg.] per century, to which should be added the correction due to Einstein, which comes to 2.34[deg.] per century. The sum, 4.27[deg.] per century, is about seven times the observed precession of 0.65[deg.] per century, they report in the August ASTRONOMICAL JOURNAL.

To the rescue may come, they say the "nonsymmetric gravity theory" of John W. Moffat of the University of Toronto (SN: 9/3/83, p. 152). Unlike Einstein's theory, Moffat's permits backward precession. The Moffat correction for DI Herculis is -1.40[deg.] per century. Combining that with the classical value yields 0.53[deg.] per century, very close to the observation.