We have obtained extensive high-quality spectroscopic observations of the
OGLE-LMC-CEP-1718 eclipsing binary system in the Large Magellanic Cloud which
Soszynski et al. (2008) had identified as a candidate system for containing two
classical Cepheids in orbit. Our spectroscopic data clearly demonstrate binary
motion of the Cepheids in a 413-day eccentric orbit, rendering this eclipsing
binary system the first ever known to consist of two classical Cepheid
variables. After disentangling the four different radial velocity variations in
the system we present the orbital solution and the individual pulsational
radial velocity curves of the Cepheids. We show that both Cepheids are
extremely likely to be first overtone pulsators and determine their respective
dynamical masses, which turn out to be equal to within 1.5 %. Since the
secondary eclipse is not observed in the orbital light curve we cannot derive
the individual radii of the Cepheids, but the sum of their radii derived from
the photometry is consistent with overtone pulsation for both variables.
The existence of two equal-mass Cepheids in a binary system having different
pulsation periods (1.96 and 2.48 days, respectively) may pose an interesting
challenge to stellar evolution and pulsation theories, and a more detailed
study of this system using additional datasets should yield deeper insight
about the physics of stellar evolution of Cepheid variables. Future analysis of
the system using additional near-infrared photometry might also lead to a
better understanding of the systematic uncertainties in current Baade-Wesselink
techniques of distance determinations to Cepheid variables.Comment: accepted to be published in Ap