Abstract
A dozen of new precise times of eclipses were measured for the eclipsing binary DX Cygni as a part of our long-term observational project for studying neglected eclipsing binaries with a short orbital period. Based on a current O - C diagram, we found for the first time that its period is increasing (dP/P = 1.68 X 10(-7) day/years) and that times of minima show also significant cyclical changes with a period of about 16 years, caused very probably by a third body orbiting the eclipsing pair.
The minimal mass of this companion is 0.49 M-circle dot. The light curve solution in PHOEBE results to the typical Algol-type semidetached configuration where the secondary fills its Roche lobe.
The temperature of primary component was fixed to T-1 = 5300 K according to its spectral type, which gives us T-2 = 3330 +/- 20 K for the secondary. The photometric mass ratio was estimated q = 0.504 +/- 0.012.
We also compare orbital parameters of selected known Algol-type eclipsing binaries with proven mass transfer and a third body.