This paper discusses the transient events resulting from neutron star mergers (NS+NS) or neutron star-stellar mass black hole mergers (NS+BH). These mergers eject a small fraction of matter with sub-relativistic velocities, leading to the formation of neutron-rich, radioactive nuclei. Radioactivity provides a long-term heat source for the expanding envelope, resulting in a brief transient with peak luminosity in the supernova range and radiation in the UV-optical domain. The authors present a simple model and analytical formulae to estimate the parameters of such transients based on poorly known input parameters. The mergers may be detected as rapid transients, many of which are far from their parent galaxies. The mysterious optical transients detected by Schmidt et al. (1998) may be related to neutron star mergers, as they typically have no visible host galaxy. The paper explores the likely light curves following NS+NS and/or NS+BH mergers, noting that the peak luminosity is similar to that of a Type Ia supernova, with the decay of $^{56}$Ni to $^{56}$Co and then to $^{56}$Fe responsible for the observed luminosity. The authors develop a 'one zone model' of an expanding envelope, assuming spherical symmetry and uniform density. They derive equations for the internal energy and luminosity of the expanding envelope, considering adiabatic expansion, radioactive heating, and radiative losses. The model shows that the peak luminosity depends on the ratio of the radioactive decay time scale to the optical depth time scale. For sub-relativistic cases, the peak luminosity reaches its maximum when the radioactive decay time scale is comparable to the optical depth time scale. The paper also discusses the implications of the model for future observations, noting that the transient events are likely to be detectable in supernova searches. The authors suggest that the high redshift supernova search by Schmidt et al. (1998) may have detected such transients, which are expected to be bright and have no host galaxy. The model provides a framework for understanding the light curves and other observable parameters of these events, and highlights the importance of future observations to confirm these predictions.This paper discusses the transient events resulting from neutron star mergers (NS+NS) or neutron star-stellar mass black hole mergers (NS+BH). These mergers eject a small fraction of matter with sub-relativistic velocities, leading to the formation of neutron-rich, radioactive nuclei. Radioactivity provides a long-term heat source for the expanding envelope, resulting in a brief transient with peak luminosity in the supernova range and radiation in the UV-optical domain. The authors present a simple model and analytical formulae to estimate the parameters of such transients based on poorly known input parameters. The mergers may be detected as rapid transients, many of which are far from their parent galaxies. The mysterious optical transients detected by Schmidt et al. (1998) may be related to neutron star mergers, as they typically have no visible host galaxy. The paper explores the likely light curves following NS+NS and/or NS+BH mergers, noting that the peak luminosity is similar to that of a Type Ia supernova, with the decay of $^{56}$Ni to $^{56}$Co and then to $^{56}$Fe responsible for the observed luminosity. The authors develop a 'one zone model' of an expanding envelope, assuming spherical symmetry and uniform density. They derive equations for the internal energy and luminosity of the expanding envelope, considering adiabatic expansion, radioactive heating, and radiative losses. The model shows that the peak luminosity depends on the ratio of the radioactive decay time scale to the optical depth time scale. For sub-relativistic cases, the peak luminosity reaches its maximum when the radioactive decay time scale is comparable to the optical depth time scale. The paper also discusses the implications of the model for future observations, noting that the transient events are likely to be detectable in supernova searches. The authors suggest that the high redshift supernova search by Schmidt et al. (1998) may have detected such transients, which are expected to be bright and have no host galaxy. The model provides a framework for understanding the light curves and other observable parameters of these events, and highlights the importance of future observations to confirm these predictions.