HST /WFPC2 observation of the core of M 3: Blue objects and the position of the radio source

HST /WFPC2 images of the central part of the globular cluster M 3 (NGC 5272) in UV (F255W), U (F336W), V (F555W) and I (F814W) are used to search for faint blue stars down to anUVmagnitude slightly below the turnoff level. Three low-luminosity blue stars are found in the PC1 field of view; two are new detections. All are within ∼ 10′′ from the cluster centre. Two are likely horizontal branch stars on the extreme blue extension which turns out very scarce in the core of this cluster. The third object has a position in the UV color-magnitude diagram consistent with the cataclysmic variables region; changes in itsmagnitude during the limited period of observation provide marginal indication of variability. One of the very few known non-pulsating radio sources, located in the direction of the centre, is accurately positioned. Its position does not match any of the low luminosity blue stars by ∼ 7′′. In turn, the error box of the radio source position contains two interesting objects: a bright horizontal branch star and a bright blue straggler, both are within ∼ 1′′. No other object in the vicinity of the radio source shows any photometric signature of an unusual cluster object or of an extragalactic source. Variability of some candidates (RR Lyrae and BSS) has been confirmed; new variable candidates of both types have also been found.