  Cool stars on the main sequence generate X-rays from coronal activity,
powered by a convective dynamo. With increasing temperature, the convective
envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I
observations of four single stars with early A spectral types. Only the coolest
star of this sample, $\tau^3$ Eri ($T_\mathrm{eff}\approx8\,,000$ K), is
detected with $\log(L_X/L_\mathrm{bol})=-7.6$ while the three hotter stars
($T_\mathrm{eff}\geq8\,,000$ K), namely $\delta$ Leo, $\beta$ Leo, and $\iota$
Cen, remain undetected with upper limits $\log(L_X/L_\mathrm{bol})<-8.4$. The
drop in X-ray emission thus occurs in a narrow range of effective temperatures
around $\sim 8100$ K and matches the drop of activity in the C III and O VI
transition region lines.
