  Until recently almost nothing was known about the evolution of magnetic
fields found in upper main sequence Ap/Bp stars during their long main sequence
lifetime. We are thus studying magnetic Ap/Bp stars in open clusters in order
to obtain observational evidence of how the properties of Ap/Bp magnetic stars,
such as field strength and structure, evolve with age during the main sequence.
One important aspect of this study is to search for the very rare examples of
hot magnetic stars in short-period binary systems among magnetic cluster
members. In this paper we characterize the object BD-19~5044L, which is both a
member of the open cluster IC 4725 = M~25, and a short-period SB2 system
containing a magnetic primary star. We have obtained a series of intensity and
circular polarisation spectra distributed through the orbital and rotation
cycles of BD-19 5044L with the ESPaDOnS spectropolarimeter at CFHT. We find
that the orbit of BD-19 5044L AB is quite eccentric (e = 0.477), with a period
of 17.63 d. The primary is a magnetic Bp star with a variable longitudinal
magnetic field, a polar field strength of ~1400 G and a low obliquity, while
the secondary is probably a hot Am star and does not appear to be magnetic. The
rotation period of the primary (5.04 d) is not synchronised with the orbit, but
the rotation angular velocity is close to being synchronised with the orbital
angular velocity of the secondary at periastron, perhaps as a result of tidal
interactions. The periastron separation is small enough (about 12 times the
radius of the primary star) that BD-19 5044L may be one of the very rare known
cases of a tidally interacting SB2 binary system containing a magnetic Ap/Bp
star.
