We report radial velocity measurements of the G-type subgiants 24 Sextanis ( =HD 90043 ) and HD 200964 .
Both are massive , evolved stars that exhibit periodic variations due to the presence of a pair of Jovian planets .
Photometric monitoring with the T12 0.80 m APT at Fairborn Observatory demonstrates both stars to be constant in brightness to \leq 0.002 mag , thus strengthening the planetary interpretation of the radial velocity variations .
24 Sex b , c have orbital periods of 452.8 days and 883.0 days , corresponding to semimajor axes 1.333 AU and 2.08 AU , and minimum masses 1.99 M _ { Jup } and 0.86 M _ { Jup } , assuming a stellar mass M _ { \star } = 1.54 M _ { \odot } .
HD 200964 b , c have orbital periods of 613.8 days and 825.0 days , corresponding to semimajor axes 1.601 AU and 1.95 AU , and minimum masses 1.99 M _ { Jup } and 0.90 M _ { Jup } , assuming M _ { \star } = 1.44 M _ { \odot } .
We also carry out dynamical simulations to properly account for gravitational interactions between the planets .
Most , if not all , of the dynamically stable solutions include crossing orbits , suggesting that each system is locked in a mean motion resonance that prevents close encounters and provides long-term stability .
The planets in the 24 Sex system likely have a period ratio near 2:1 , while the HD 200964 system is even more tightly packed with a period ratio close to 4:3 .
However , we caution that further radial velocity observations and more detailed dynamical modelling will be required to provide definitive and unique orbital solutions for both cases , and to determine whether the two systems are truly resonant .