HST imaging and light curves of 21 SPIRITS variables

HST images show most of these cool and dusty variables are associated with star-forming regions in late-type galaxies, implying an origin in massive stars. Two SPRITEs lacked optical progenitors in deep preoutburst HST images; however, one was detected during eruption at J and H, indicating a dusty object with an effective temperature of ~1050K. One faint SPRITE turned out to be a dusty classical nova.

About half the HST targets proved to be periodic variables, with pulsation periods of 670-2160d; they are likely dusty asymptotic-giant-branch (AGB) stars with masses of ~5-10M_{sun}_. A few of them were warm enough to be detected in deep HST frames, but most are too cool. Out of six irregular variables, two were red supergiants with optical counterparts in HST images; four were too enshrouded for HST detection.