We present the Fundamental Plane ( FP ) in the z = 0.28 cluster of galaxies RX J0142.0+2131 .
There is no evidence for a difference in the slope of the FP when compared with the Coma cluster , although the internal scatter is larger .
On average , stellar populations in RX J0142.0+2131 have rest-frame V -band mass-to-light ratios ( M / L _ { V } ) 0.29 \pm 0.03 dex lower than in Coma .
This is significantly lower than expected for a passively-evolving cluster formed at z _ { f } = 2 .
Lenticular galaxies have lower average M / L _ { V } and a distribution of M / L _ { V } with larger scatter than ellipticals .
Lower mass-to-light ratios are not due to recent star formation : our previous spectroscopic observations of RX J0142.0+2131 E/S0 galaxies showed no evidence for significant star-formation within the past \sim 4 Gyr .
However , cluster members have enhanced \alpha -element abundance ratios , which may act to decrease M / L _ { V } .
The increased scatter in the RX J0142.0+2131 FP reflects a large scatter in M / L _ { V } implying that galaxies have undergone bursts of star formation over a range of epochs .
The seven easternmost cluster galaxies , including the second brightest member , have M / L _ { V } consistent with passive evolution and z _ { f } = 2 .
We speculate that RX J0142.0+2131 is a cluster–cluster merger where the galaxies to the east are yet to fall into the main cluster body or have not experienced star formation as a result of the merger .