Does Collinder 236 host a Cepheid calibrator

Photoelectric UBV photometry and star countsare presented for the previously unstudied open clusterCollinder 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 (3.2 ± 1.1) × 10 7 yr, 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 r n ≃ 2 arcmin (1.1 pc) and R c ≃ 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 five 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.