The I-Z colour has been recently shown to be a good temperature indicator for M dwarfs. We present the first IZ photometry of a small sample of L dwarfs ranging in spectral type from L0.5V to L6.0V. We find that the I-Z colour is not a good temperature indicator for objects between L1V and L5V, such objects having colours that overlap with mid M dwarfs. We attribute this to the reduction in the strength of the TiO and VO bands in the L dwarfs which are the dominant opacity source in the I band for late M dwarfs. Beyond L5V, I-Z appears to be a reasonable indicator. This has important implications for the planning of optical surveys for cool objects in clusters and the field. For example I-Z will cease to be a good method of discriminating brown dwarfs in the Pleiades below around 0.04 Msolar, and at around 0.075 Msolar in the Hyades and Praesepe.
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