The definitions of the basic concepts, rules, and constraints of centering theory involve underspecified notions such as 'previous utterance', 'realization', and 'ranking'. We attempted to find the best way of defining each such notion among those that can be annotated reliably, and using a corpus of texts in two domains of practical interest. Our main result is that trying to reduce the number of utterances without a backward-looking center (CB) results in an increased number of cases in which some discourse entity, but not the CB, gets pronominalized, and viceversa.

In this paper we discuss some problems arising in German-Russian Machine Translation with regard to tense and aspect. Since the formal category of aspect is missing in German the information required for generating Russian aspect forms has to be extracted from different representation levels. A sentence based procedure for aspect choice in the MT system VIRTEX is presented which takes lexical, morphological and semantic criteria into account. The limits of this approach are shown. To overcome these difficulties a human interaction component is proposed.