Which tonnage has a Date of 26 september 1940, and a Ship Name of ashantian?
CREATE TABLE "summary_of_raiding_history" (
  "date" text,
  "ship_name" text,
  "flag" text,
  "tonnage_grt" real,
  "fate" text,
  "deaths" real
);
SELECT MIN("tonnage_grt") FROM "summary_of_raiding_history" WHERE "date"='26 september 1940' AND "ship_name"='ashantian';