We present here the first spectroscopic and photometric analysis of the double-lined eclipsing binary containing the classical , first-overtone Cepheid OGLE-LMC-CEP-2532 ( MACHO 81.8997.87 ) .
The system has an orbital period of 800 days and the Cepheid is pulsating with a period of 2.035 days .

Using spectroscopic data from three high-class telescopes and photometry from three surveys spanning 7500 days we are able to derive the dynamical masses for both stars with an accuracy better than 3 \% .
This makes the Cepheid in this system one of a few classical Cepheids with an accurate dynamical mass determination ( M _ { 1 } = 3.90 \pm 0.10 M _ { \odot } ) .
The companion is probably slightly less massive ( 3.82 \pm 0.10 M _ { \odot } ) , but may have the same mass within errors ( M _ { 2 } / M _ { 1 } = 0.981 \pm 0.015 ) .
The system has an age of about 185 million years and the Cepheid is in a more advanced evolutionary stage .

For the first time precise parameters are derived for both stars in this system .
Due to the lack of the secondary eclipse for many years not much was known about the Cepheid ’ s companion .
In our analysis we used extra information from the pulsations and the orbital solution from the radial velocity curve .
The best model predicts a grazing secondary eclipse shallower than 1 mmag , hence undetectable in the data , about 370 days after the primary eclipse .

The dynamical mass obtained here is the most accurate known for a first-overtone Cepheid and will contribute to the solution of the Cepheid mass discrepancy problem .