We present optical photometry ( i - and Z -band ) and low resolution spectroscopy ( 640–1015 nm ) of very faint candidate members ( J = 20.2 - 21.2 mag ) of the Pleiades star cluster ( 120 Myr ) .
The main goal is to address their cluster membership via photometric , astrometric , and spectroscopic studies , and to determine the properties of the least massive population of the cluster through the comparison of the data with younger and older spectral counterparts and state-of-the art model atmospheres .
We confirm three bona-fide Pleiades members that have extremely red optical and infrared colors , effective temperatures of \approx 1150 K and \approx 1350 K , and masses in the interval 11–20 M _ { Jup } , and one additional likely member that shares the same motion as the cluster but does not appear to be as red as the other members with similar brightness .
This latter object requires further near-infrared spectroscopy to fully address its membership in the Pleiades .
The optical spectra of two bona-fide members were classified as L6–L7 and show features of K i , a tentative detection of Cs i , hydrides and water vapor with an intensity similar to high-gravity dwarfs of related classification despite their young age .
The properties of the Pleiades L6–L7 members clearly indicate that very red colors of L dwarfs are not a direct evidence of ages younger than \approx 100 Myr .
We also report on the determination of the bolometric corrections for the coolest Pleiades members .
These data can be used to interpret the observations of the atmospheres of exoplanets orbiting stars .