Far-red optical colours of late-M and L dwarfs

We present both synthetic and observed I - Z colours for a sample of spectroscopically classified M and L dwarfs drawn primarily from recent wide-field infrared surveys. Our synthetic photometry does not reproduce the strong decrease in the I-Harris - Z(RGO) natural colour beyond M9 to a minimum around L2.5 reported by Steele & Howells. However, it does support the result that the - ZRGO colour does not increase with spectral type between L0-L2.5. Instead, the I-Harris - Z(RGO) i(WFC) - Z(WFC), i(SDSS) - z(SDSS) and I-C - Z(RGO) colours are shown to remain more or less constant before increasing once again for types later than L3-L4. These results, which are supported by both previously published observations and new photometry obtained with the Isaac Newton Telescope (INT) and the Wide Field Camera (WFC), have implications for the interpretation of the recently publicly released Wide Field Survey Programme data covering the Pleiades cluster and surveys for late-type M and L dwarfs in general.