The Color-Magnitude Diagram of NGC 2264
  Existing photometry for NGC 2264 tied to the Johnson and Morgan (1953) UBV
system is reexamined and, in the case of the original observations by Walker
(1956), reanalyzed in order to generate a homogeneous data set for cluster
stars. Color terms and a Balmer discontinuity effect in Walker's observations
were detected and corrected, and the homogenized data were used in a new
assessment of the cluster reddening, distance, and age. Average values of
E(B-V)=0.075+-0.003 s.e. and Vo-Mv=9.45+-0.03 s.e. (d=777+-12 pc) are obtained,
in conjunction with an inferred cluster age of ~5.5x10^6 yr from
pre-main-sequence members and the location of the evolved, luminous, O7 V((f))
dwarf S Mon relative to the ZAMS. The cluster main sequence also contains gaps
that may have a dynamical origin. The dust responsible for the initial
reddening towards NGC 2264 is no more than 465 pc distant, and there are
numerous, reddened and unreddened, late-type stars along the line of sight that
are difficult to separate from cluster members by standard techniques, except
for a small subset of stars on the far side of the cluster embedded in its gas
and dust and background B-type ZAMS members of Mon OB2. A compilation of likely
NGC 2264 members is presented. Only 3 of the 4 stars recently examined by
asteroseismology appear to be likely cluster members. NGC 2264 is also noted to
be a double cluster, which has not been mentioned previously in the literature.
