  A preliminary determination of the first moment of the hadronic mass
distribution <(M_X)^2 - bar{m_D}^2> in semileptonic B decays has been obtained
as a function of the minimum lepton momentum, ranging from 0.9 to 1.6 GeV/c.
The measurement is based on a new technique involving BBbar events in which one
fully reconstructed B meson decays hadronically and the recoiling B decays
semileptonically. The mass of the hadrons in the semileptonic decay is
determined from a kinematic fit to the whole event. For different minimum
lepton momenta, the mass distribution is decomposed into contributions from
various charm resonant states and a non-resonant contribution, allowing for the
determination of the first moment. From these moments the Heavy Quark Effective
Theory (HQET) parameters lambda_1 and Lambdabar can be derived. For lepton
momenta in the B rest frame above 1.5 GeV/c, we find a first moment that is
compatible with existing measurements. However, if we extend the measurement to
lower values of lepton momenta, the data can only be described by Operator
Product Expansion calculations if we use significantly different values for
Lambdabar and lambda_1 than obtained from earlier measurements based on lepton
momentum spectra and the photon spectrum in b --> s gamma transitions.
