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The globular cluster Messier 80 was monitored by the Kepler space telescopefor 80 days during the K2 mission. Continuous, high-precision photometry ofsuch an old, compact cluster allows us to study its variable star population inunprecedented detail. We extract light curves for 27 variable stars usingdifferential-image photometry. A search for new variables in the images led tothe discovery of two new variable stars: an RR Lyrae and a variable red giantstar, respectively. Analysis of the RR Lyrae population reveals multiple RRcstars with additional modes and/or peculiar modulation cycles. We newlyclassify star V28 as a spotted extreme horizontal branch variable. Despitetheir faintness, we clearly detect the three SX Phe stars but we did not findnew pulsation modes beyond the known ones in them. Spectra taken with the VLTand Magellan Clay telescopes, as well as absolute color-magnitude diagrams ofthe cluster based on Gaia and Pan-STARRS observations confirm theclassification of the peculiar modulated variables as bona-fide RRc stars. Wepropose that they highlight a subgroup of overtone stars that may have beenoverlooked before. We fit MESA isochrones to the CMDs to estimate the age andmetallicity of the cluster. We confirm that M80 is old and metal-poor, but showthat isochrone fitting to old populations comes with numerous uncertainties.

To Grow Old and Peculiar: A Survey of Anomalous Variable Stars in M80 and Age Determination using K2 and Gaia