White dwarfs are generally too cool to have their binary properties readily measured. For example, the binary properties of double degenerate cataclysmic variables of the AM CVn type are determined from observing the hot spot where the accreting matter from the donor dwarf collides with the accretion disk around the accretor (see Warner  and references therein). With an expected , these objects would be virtually invisible at globular cluster distances. Searches for cataclysmic variables generally focus on low-luminosity X-ray sources [87, 64, 159] and on ultraviolet-excess stars [62, 93, 103], but these systems are usually a white dwarf accreting from a low mass star. The class of “non-flickerers” which have been detected recently [26, 156] have been explained as He white dwarfs in binaries containing dark CO white dwarfs [41, 65].
Pulsars, although easily seen in radio, are difficult to detect when they occur in hard binaries, due to the Doppler shift of the pulse intervals. Thanks to an improved technique known as an “acceleration search” , which assumes a constant acceleration of the pulsar during the observation period, more short orbital period binary pulsars are being discovered [20, 21, 29, 30, 48, 51, 130]. For a good review and description of this technique, see Lorimer . The progenitors of the ultracompact millisecond pulsars are thought to pass through a low-mass X-ray binary (LMXB) phase [37, 64, 131, 134]. These systems are very bright and all of them in the globular cluster system are known. There may, however, be some additional LMXBs that are currently quiescent . In addition, a very recent observation has shown that the LMXB in M15 is, in fact, two bright sources .
With the exception of M15 [53, 66], no black holes have been identified in globular clusters. Theoretical predictions of black holes in globular clusters indicate that there may be intermediate-mass black holes () in as many as globular clusters , or that stellar mass binary black holes may be generated and subsequently ejected from most globular clusters . If the velocity dispersion in globular clusters follows the same correlation to black hole mass as in galactic bulges, then there may be black holes with masses in the range in many globular clusters .
Recent observations and catalogs of known binaries are presented in the following sections.