The exploitation of the CoRoT treasure of stars observed in the exoplanetary
field allowed the detection of a unusual triple-mode Cepheid in the Milky Way,
CoRoT 0223989566. The two modes with the largest amplitudes and period ratio of
0.80 are identified with the first (P1=1.29 d) and second (P2=1.03 d) radial
overtones. The third period, which has the smallest amplitude but able to
produce combination terms with the other two, is the longest one (P3=1.89 d).
The ratio of 0.68 between the first-overtone period and the third period is the
unusual feature. Its identification with the fundamental radial or a nonradial
mode is discussed with respect to similar cases in the Magellanic Clouds. In
both cases the period triplet and the respective ratios make the star unique in
our Galaxy. The distance derived from the period-luminosity relation and the
galactic coordinates put CoRoT~0223989566 in the metal-rich environment of the
"outer arm" of the Milky Way.Comment: Published as ApJ Lette
