Sm. Rucinski, ECLIPSING BINARIES IN THE OGLE VARIABLE-STAR CATALOG - III - LONG-PERIOD CONTACT SYSTEMS, The Astronomical journal, 115(3), 1998, pp. 1135-1144
A sample of contact binaries discovered by the OGLE project in Baade's
window, with orbital periods longer than 1 day and with available col
or and light-curve data, has been analyzed. It consists of only 32 sys
tems, in contrast to 388 W Ursae Majoris type systems with shorter per
iods that were analyzed before. Most systems are very distant and are
probably located close to or in the Galactic bulge. Two groups of cont
act binaries are seen in the sample: (1) a continuation of the W UMa t
ype sequence, extending up to the orbital periods of 1.3-1.5 days, but
rather sharply ending in this period range; and (2) an inhomogeneous
group of rare systems with long periods up to 26 days, all with red co
lors and relatively shallow eclipses. While the systems of the first g
roup share most of the characteristics of the typical W UMa type syste
ms (except that they are on the average brighter and more distant, and
hence more reddened), the long-period systems do not seem to form an
early-type extension of contact binaries, but may consist of a mixture
of late-type objects, including tidally distorted red giants with inv
isible companions.