Beta Lyra, also called Sheliak, has a magnitude range from 3.4 at maximum, to two minima at +3.8 and +4.1 magnitudes with a period of 12.9138 days. It is located about 860 light years away according to Helmut Abt and his coworkers in a 1962 publication. The primary is a B6-type class II or III giant about 3000 times as bright as the Sun, with a mass estimated to be about 12 times the Sun. Its companion has not been directly detected because it seems to be enshrouded by an accretion disk fed by the primary star. Both stars are football-shaped because of their close proximity. The light variation depends mostly on the changing surface areas of the elongated stars as they orbit one another in almost direct contact. They are about 22 million miles apart, give or take. The flow of matter between the primary and the secondary can be detected spectroscopically. Eventually the secondary star will evolve into a giant and outshine the primary star if it is not already a white dwarf or even a black hole!
The formula for predicting the dates of photographic minimum has been computed by astronomers Petr Harmansee and Gerhard Scholz from several thousand eclipses of these stars measured since 1784. They get ( See Acta Astronomica, vol 23, p. 331 published in 1993)
T = 2408247.953 + 12.9138 x E + 0.00000388 x E^2Here is a table of minima beginning with cycle 3248:
Cycle.......Julian Day.........Date 3248 2450232.9 May 29 ( JD = 2450232.5) 3249 2450245.8 June 11 (JD = 2450245.5) 3250 2450258.8 June 25 (JD = 2450258.5) 3251 2450271.7 July 5 (JD = 2450271.5)I think these are pretty accurate.