New constraints on time-reversal asymmetry from a search for a permanent electric dipole moment of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mmultiscripts><mml:mrow><mml:mi mathvariant="normal">Hg</mml:mi></mml:mrow><mml:mprescripts/><mml:mrow/><mml:mrow><mml:mn>199</mml:mn></mml:mrow><mml:mrow/><mml:mrow/></mml:mmultiscripts></mml:mrow></mml:math>

A search for permanent electric dipole moment of $^{199}\mathrm{Hg}$ atoms has yielded the null result d${(}^{199}$Hg)=(0.7\ifmmode\pm\else\textpm\fi{}1.5)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}26}$ e\ensuremath{\cdot}cm, which improves by an order of magnitude the limits on several possible interactions that violate time-reversal symmetry. The experiment was performed by the use of optically pumped atomic oscillators to measure any shift in the NMR frequency of $^{199}\mathrm{Hg}$ (I=(1/2) produced by an external electric field.