CCD photometry of the globular cluster NGC 5986 and its posteasymptotic giant branch and RR Lyrae stars

We have obtained new CCD BV photometry of the little-studied southern Galactic globular cluster NGC 5986, including light curves of five of its RR Lyrae variables. The cluster's red giant branch bump is detected for the first time, at V = 16.47 +/- 0.03. We derive a reddening and true distance modulus of E(B-V) = 0.29 +/- 0.02 and (m - M)(0) = 15.15 +/- 0.10, respectively. The cluster's color-magnitude diagram reveals a mostly blue horizontal branch, like that of M13 or M2, and quite unlike M3; yet all of these clusters have nearly identical metallicities ([Fe/H](CG97) = -1.35). We show that the RR Lyrae variables in NGC 5986 are about 0.2 mag brighter on average than those in M3, an important exception to the often-employed, universal M-V(RR)-[Fe/H] relation. Finally, we note that NGC 5986 contains two luminous stars with spectral types A-F, which are likely to be post-asymptotic giant branch (PAGB) objects. The V-band luminosity function of such yellow PAGB stars is a promising standard candle. We suggest that the luminosity function is sharply peaked at M-V (PAGB) = -3.28 +/- 0.07.