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Summary form only given. Since the discovery of high transition temperature superconductivity more than decade ago a controversial debate still exists on the underlying pairing mechanism not yet resolved. A key parameter is the carrier concentration in the conducting copper-oxide planes, which, in the case of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO), is varied by altering the oxygen content from the under-doped, via the optimally-doped (i.e., highest T/sub c/) to the over-doped region. Below T/sub c/ in optimally-doped materials, the largest changes of the (ab)-plane reflectivity R(/spl omega/,T) appear in the mid-infrared spectral range around /spl plusmn//spl omega//spl ap/100 meV, which are directly connected to the opening of a gap for electronic transitions near the Fermi level. In underdoped cuprates, these features ("pseudogap") already occur at temperatures T* above T/sub c/. We present first experiments probing this mid-infrared range in optimally (T/sub c/=88 K) and under-doped (T/sub c/=68 K) YBCO on ultrafast time scales.