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We have performed targeted searches of known, extragalactic transient eventsat millimetre wavelengths using nine seasons (2013--2021) of 98, 150, and 229GHz Atacama Cosmology Telescope (ACT) observations that mapped ${\sim}40$ percent of the sky for most of the data volume. We observe at least once 88gamma-ray bursts (GRBs), 12 tidal disruption events (TDEs) and 203 othertransients, including supernovae (SNe). We stack our ACT observations toincrease the signal-to-noise ratio of the maps. In all cases but one, we do notdetect these transients in the ACT data. The single candidate detection (eventAT2019ppm), seen at ${\sim}5\sigma$ significance in our data, appears to be dueto active galactic nuclei (AGN) activity in the host galaxy coincident with atransient alert. For each source in our search we provide flux upper limits.For example, the medians for the 98 GHz 95 per cent confidence interval upperlimits are $28$, $15$, and $16$ mJy for GRBs, SNe, and TDEs respectively. Thesensitivity of future wide-area cosmic microwave background (CMB) surveysshould be good enough to detect many of these events using the methodsdescribed in this paper.

The Atacama Cosmology Telescope: Flux Upper Limits from a Targeted Search for Extragalactic Transients