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We determine the top quark mass m t using t t pairs produced in the DO " detector by ͱsϭ1.8 TeV pp collisions in a 125 pb Ϫ1 exposure at the Fermilab Tevatron. We make a two constraint fit to m t in t t→bW ϩ b W Ϫ final states with one W boson decaying to qq and the other to e or . Likelihood fits to the data yield m t (lϩjets)ϭ173.3Ϯ5.6 (stat) Ϯ 5.5 (syst) GeV/c 2 . When this result is combined with an analysis of events in which both W bosons decay into leptons, we obtain m t ϭ172.1Ϯ5.2 (stat) Ϯ 4.9 (syst) GeV/c 2 . An alternate analysis, using three constraint fits to fixed top quark masses, gives m t (lϩjets)ϭ176.0 Ϯ7.9 (stat)Ϯ 4.8 (syst) GeV/c 2 , consistent with the above result. Studies of kinematic distributions of the top quark candidates are also presented. ͓S0556-2821͑98͒06815-5͔

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We have searched for central production of a pair of photons with high transverse energies in pp collisions at √ s = 1.8 TeV using 70 pb −1 of data collected with the DØ detector at the Fermilab Tevatron in 1994-1996. If they exist, virtual heavy pointlike Dirac monopoles could rescatter pairs of nearly real photons into this final state via a box diagram. We observe no excess of events above background, and set lower 95% C.L. limits of 610, 870, or 1580 GeV/c 2 on the mass of a spin 0, 1/2, or 1 Dirac monopole.