Decomposing the 13 May 2026 duty jump

On 12 and 13 May 2026 the Yahoo daily closes were: COMEX gold (GC=F) $4,686.70 and $4,706.70 per troy ounce; USDINR (INR=X) 95.3914 and 95.6330; GOLDBEES ₹124.61 and ₹131.74. The gold import duty (BCD plus AIDC) was 6% on 12 May and 15% from 13 May. Use 1 troy ounce $= 31.1034768$ g.

(a) Compute the duty-paid import parity (10.15), with $q = 0$ and before GST, in rupees per gram on each day. (b) Decompose the log change in parity into its gold, rupee and duty terms using (10.16), and check that they sum to the total. What is the simple percentage jump that the duty term alone implies? (c) Compute the log change in GOLDBEES and the log change in its premium or discount to parity, $\ln(u_1/u_0)$ with $u = C/\Pi$. (d) An investor held ₹3,00,000 of GOLDBEES at the 12 May close, as a whole number of units. What was the rupee gain to the 13 May close, and what would it have been had the ETF moved exactly with parity? (e) In two or three sentences: could a rule-based trader have positioned for this move, and how should an Indian gold position be sized given that such moves exist?