Doubly heavy $QQ$ tetraquarks

With the discovery of a doubly charmed ${\mathrm{\Xi }}_{cc}$ baryon a somewhat forgotten issue of tetraquarks containing two heavy and two light (anti) quarks, ${\cal T}{QQ}$,triggeredtheoris{t}^{\mathrm{\prime }}sinterest\mathrm{.}Wediscussquarkmodelestimatesof${\cal T}{QQ}$ masses and such objects including a model where the light sector is treated as a soliton. We show that this model has different large $N_c$ limit than other approaches.