  We have analyzed the double-lined eclipsing binary system OGLE-LMC-CEP-1812
in the LMC and demonstrate that it contains a classical fundamental mode
Cepheid pulsating with a period of 1.31 days. The secondary star is a stable
giant. We derive the dynamical masses for both stars with an accuracy of 1.5%,
making the Cepheid in this system the second classical Cepheid with a very
accurate dynamical mass determination, following the OGLE-LMC-CEP-0227 system
studied by Pietrzynski et al. (2010). The measured dynamical mass agrees very
well with that predicted by pulsation models. We also derive the radii of both
components and accurate orbital parameters for the binary system. This new,
very accurate dynamical mass for a classical Cepheid will greatly contribute to
the solution of the Cepheid mass discrepancy problem, and to our understanding
of the structure and evolution of classical Cepheids.
