2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/50979New exact string solutions in non-constant background fields are found and it is shown that some of them are compatible with the boundary conditions for the open string - D-brane system. Extension of the constraint algebra is proposed and discussed.LaTeX, 10 pages, no figures; V2: minor improvements in the text, references rewritten, report-no addedHigh Energy Physics - TheoryExact String Solutions and the Open String - D-Brane System in Non-Constant Background Fields Itext