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In the interbank foreign
exchange market, options are not quoted with prices. They are quoted indirectly
with implied volatilities. The convention for converting volatilities to prices
is the Garman and Kohlhagen
(1983)
option pricing formula.
Mathematically, the formula is identical to
Merton's (1973) formula for options
on dividend-paying
stocks. Only the term q,
which did represent a stock's dividend yield, now represents the foreign currency's
continuously compounded
risk-free rate. Like the Merton formula, the Garman and Kohlhagen
formula applies only to European options. Generally,
OTC currency options are
European.
Values for a
call price
c or put price p are:
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[2] |
where:
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[3] |
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[4] |
Here, log denotes the natural logarithm, and:
s = the current exchange rate (domestic
currency per unit of foreign currency)
x = the
strike exchange rate
r = the continuously compounded domestic risk free interest rate
q = the continuously compounded foreign risk free interest rate
t = the time in years until the
expiration of the
option
σ = the implied volatility for
the underlying exchange rate
Φ = the
standard normal
cumulative distribution function.
Consider a put GBP call USD option on GBP10MM for which we want
a USD value. The option is struck at 1.65 USD/GBP and expires in .09 years. The
current exchange rate is 1.62 USD/GBP. Assume 18% (that is .18) implied volatility.
USD and GBP continuously compounded risk free interest rates are .0294 and
.0327. Applying
formula [2], the option has
market value USD .0524 per GBP. Based upon the
notional amount of GBP 10MM, that becomes USD 0.524MM. Because the option is
out-of-the-money, that value is entirely
time value.
Because their prices are
affected by two—one domestic and the other foreign—risk free rates, currency
options have two rho sensitivities. All the Greeks—delta, gamma, vega, theta,
domestic rho and foreign rho—for a call are:
where
denotes the standard
normal probability density function. For a put, the Greeks are:
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[12] |
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[13] |
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[14] |
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[15] |
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[16] |
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Haug (1997)
is a handy encyclopedia of published option pricing formulas.
Walmsley (2000)
is a sophisticated book on the foreign exchange markets.
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Black, Fischer and Myron S. Scholes
(1973). The pricing of options and corporate liabilities, Journal
of Political Economy, 81, 637-654.
Garman, Mark B. and Steven W.
Kohlhagen (1983). Foreign currency option values, Journal of
International Money and Finance, 2, 231-237.
Merton, Robert C. (1973). Theory of
rational option pricing, Bell Journal of Economics and
Management Science, 4 (1), 141-183. Available in Merton (1992).
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