Does Collinder 236 host a Cepheid calibrator?
  Photoelectric UBV photometry and star counts are presented for the previously
unstudied open cluster Collinder 236, supplemented by observations for stars
near the Cepheid WZ Car. Collinder 236 is typical of groups associated with
Cepheids, with an evolutionary age of (3.4+-1.1)x10^7 years, but it is 1944+-71
pc distant, only half the predicted distance to WZ Car. The cluster is reddened
by E(B-V)~0.26, and has nuclear and coronal radii of rn~2 arcmin (1.1 pc) and
Rc~8 arcmin (4.5 pc), respectively. The Cepheid is not a member of Collinder
236 on the basis of location beyond the cluster tidal radius and implied
distance, but its space reddening can be established as E(B-V)=0.268+-0.006
s.e. from 5 adjacent stars. Period changes in WZ Car studied with the aid of
archival data are revised. The period of WZ Car is increasing, its rate of
+8.27+-0.19 s yr^(-1) being consistent with a third crossing of the instability
strip.
