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The First Orbital Period for a Dwarf Nova in a Globular Cluster: V101 in M5

We report the first orbital period determination for a Dwarf Nova (DN) in a glubular cluster: V101 in M5 has a period of 5.796 +- 0.036 hours. We derived this period from I-band photometry acquired with the Calypso Observatory High Resolution Camera operating with tip-tilt adaptive optics correction. Observations from the South African Astronomical Observatory in the V-band were also analyzed and exhibit a periodic signal of the same period. This orbital period suggests that V101 has a secondary of mid to late K spectral type with Mv = +8.2 +- 0.5. The predicted spectral type is consistent with previous spectral observations in quiescence which show a fairly red continuum. From the observed minimum brightness of V = 22.5, we derive a distance modulus of (m - M)v = 14.3 +- 0.5 to the DN which supports V101's membership in the globular cluster M5. Measurement of the ellipsoidality effect indicates that the orbital plane of the V101 system is moderately inclined, but not enough to exhibit eclipses.