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Limits on charged-scalar Yukawa couplings from r and B decays are discussed. They (and other existing limits) are consistent with "strong" couplings (-1 for the third generation) even if the lightest scalar mass is in the range 20 GeV ;SAf^;S 100 GeV but saturated in this case. B(B-» xvX) and (for M $ > m t ) B(T-* rvX) may be then as large as 30% and 70%, respectively. Both for M^ m t the potentially possible top-quark signature in pp collisions is T+2 jets final state. The upper limit for T-• vi\n is ==0.003%. PACS numbers: 12.15.Cc, 13.20.Jf, 13.35,+s, 14.80.Gt In most "beyond the standard" models several Higgs doublets are present and consequently the weak forces are mediated, in addition to the intermediate vector bosons, by charged scalar particles. Models with two and three scalar doublets have been explicitly studied and some constraints on the quark_Yukawa couplings have been derived from K°-K°, D°-D°, and B°-B° mixing 1 ' 2 and from CP nonconservation. 3 One appealing possibility is that in three-(or more-) doublet models the hierarchy of the vacuum expectation values is such that the Yukawa couplings are of the same order for the members of a given heavy generation and -~ 1 for the third generation (we shall call such couplings "strong").On the phenomenological side a recent thorough analysis 4 of the high-precision data on muon decay, inverse muon decay, nn decays, and nuclear GamowTeller transitions shows that in those reactions the effective scalar coupling might be of the order of 10% of GF, provided it is proportional to the lepton mass.In this paper we analyze the limits on the chargedscalar couplings from the T and B decays. In three-(or more-) doublet models with "natural" absence at the tree level of flavor-changing neutral currents there are two independent sets of Yukawa couplings, driving the up-and down-quark masses, respectively. Our bounds are consistent with both being -1 for the third generation and they are in fact saturated by such strong couplings if the lightest scalar mass is in the range 20 GeV ;SM^;S100 GeV. In this case the branching ratios B(B-* rvX) and (for M, > m t ) £(7-rvX) may be as large as 30% and 70%, respectively. Thus, those channels are very restrictive for potential scalar exchange.r decays.-Our notation (for effective couplings) is shown in Fig. 1. We also define the effective scalar couplings G,and the ratios //,As has already been mentioned, from the e-p sector one gets the limit 4 H e^< 0.22provided that Y e =m e hi, K^-m^/u/.With scalar exchange included, the rate for the decay N v FIG 1. Our notation for effective couplings.182

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