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 ( P _ { 1 } =1.29 d ) and second ( P _ { 2 } =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 ( P _ { 3 } =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 .