HST UV Observations of the Cores of M3 and M13

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We present preliminary results from HST/WFPC2 observations of the central regions of the of the Galactic globular clusters M13 and M3. The clusters are almost identical in most respects including chemical composition, but there are dramatic differences in both the horizontal branch (HB) and blue straggler (BSS) populations. The M13 HB has a long blue tail extending 4.5 mag in V, reaching well below the level of the main sequence turn-off. M3 has no such feature. M3 and M13 are thus an extreme case of the ``second parameter problem'' in HB morphology. Also present in the M13 HB are two gaps similar to those seen in the clusters NGC 6752 and NGC 2808. M3 has a specific frequency of BSS three times larger than that of M13. Our results imply that neither age nor cluster density, two popular second parameter candidates, are likely to be responsible for the observed differences.