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Red Variable Stars in Three M81 Group Dwarf Galaxies

Archival [3.6] and [4.5] images are used to identify and characterize variable stars in the Magellanic-type galaxies Holmberg II, NGC 2366, and IC 2574. Using parametric and non-parametric detection methods, 74 confirmed or suspected long period variables (LPVs) are found. The period distributions of the LPVs in NGC 2366 and IC 2574 are similar. While the period distribution of LPVs in Ho II is uncertain due to small number statistics there appears to be a deficiency of LPVs with periods between 550 and 650 days when compared with NGC 2366 and IC 2574. The LPVs are diffusely distributed on the sky, and do not follow the underlying light from unresolved stars, as expected if episodes of star formation within the past few hundred Myr have occurred throughout the galaxies, including their outer regions. Distances computed for Ho II and NGC 2366 from the period-luminosity relations (PLRs) agree to within ~0.1 magnitudes with those based on the tip of the red giant branch (RGB). Efforts to estimate an LPV-based distance modulus for IC 2574 are complicated by the presence of first overtone pulsators among LPVs with periods ~ 600 days, although the PLR at the long period end is consistent with the distance estimated from the RGB-tip. In addition to the LPVs, 10 candidate sgB[e] or luminous blue variables and 2 candidate red supergiant variables are also identified. Nine candidate sgB[e] stars that do not show evidence of variability are also identified based on their locations in the color-magnitude diagram.