We have identified three K2 transiting star-planet systems , K2-51 ( EPIC 202900527 ) , K2-67 ( EPIC 206155547 ) , and K2-76 ( EPIC 206432863 ) , as stellar binaries with low-mass stellar secondaries .
The three systems were statistically validated as transiting planets , and through measuring their orbits by radial velocity monitoring we have derived the companion masses to be 0.1459 ^ { +0.0029 } _ { -0.0032 } M _ { \sun } ( EPIC 202900527 B ) , 0.1612 ^ { +0.0072 } _ { -0.0067 } M _ { \sun } ( EPIC 206155547 B ) , and 0.0942 \pm 0.0019 M _ { \sun } ( EPIC 206432863 B ) .
Therefore they are not planets but small stars , part of the small sample of low-mass stars with measured radius and mass .
The three systems are at an orbital period range of 12–24 days , and the secondaries have a radius within 0.9–1.9 R _ { J } , not inconsistent with the properties of warm Jupiter planets .
These systems illustrate some of the existing challenges in the statistical validation approach .
We point out a few possible origins for the initial misclassification of these objects , including poor characterization of the host star , the difficulty in detecting a secondary eclipse in systems on an eccentric orbit , and the difficulty in distinguishing between the smallest stars and gas giant planets as the two populations have indistinguishable radius distributions .
Our work emphasizes the need for obtaining medium-precision radial velocity measurements to distinguish between companions that are small stars , brown dwarfs , and gas giant planets .